The central facts are: Comey openly leaked classified information to get a special counsel. Rosenstein WROTE A DAMNED MEMO, for heaven's sake, recommending that Comey be fired. Then, when the president acted on that recommendation, Rosenstein promptly appointed Mueller. That's as dirty as it gets.
You conveniently overlook a critical series of FACTS here.
Sessions and Rosenstein had documented the reasons for Comey to be fired, and then -- for reasons I'll still never understand -- President Trump decided to do an interview with a sh!tbag news outlet like NBC and tell Lester Holt that he was going to fire Comey anyway over the "Russian collusion" nonsense. NBC was all too happy to help Trump undermine himself and make himself look like he had something to hide.
The made it look like Rosenstein and Trump colluded to fire Comey for nefarious reasons and then have Rosenstein lie about it in his formal memo. At this point Rosenstein had no choice but to get someone else involved, since he clearly had a potential conflict of interest in the case.
Why would President Trump do a stupid interview for a news outlet (NBC) that he'd spent the prior 18 months denigrating as a source of biased/phony news?